The Circulatory System, the Heart, and Blood - The heart at work

The structure and performance of the heart, at first glance rather
complicated, assume a magnificent simplicity once we observe that this
pulsating knot of hollow, intertwining muscle uses only one beat to
perform two distinct pumping jobs.

The heart has a right side (your right) and a left side (your left),
divided by a tough wall of muscle called a
septum
. Each side has two chambers, an upper one called an
atrium
(or
auricle)
, and a lower one called a
ventricle
.

How the Heart Pumps the Blood

Venous blood from the body flows into the right atrium via two large
veins called the
superior vena cava
(bringing blood from the upper body) and the
inferior vena cava
(bringing blood from the lower part of the body). Where the blood
enters the right atrium are valves that close when the atrium chamber is
full.

Then, through a kind of trapdoor valve, blood is released from the right
atrium into the right ventricle. When the right ventricle is full, and
its outlet valve opens, the heart as a whole contracts—that is,
pumps.

To the Lungs

The blood from the right ventricle is pumped to the lungs through the
pulmonary artery to pick up oxygen. The trapdoor valve between the right
atrium and ventricle has meanwhile closed, and venous blood again fills
the right atrium.

Having picked up oxygen in the lungs, blood enters the left atrium
through the right and left pulmonary veins. (They are called veins
despite the fact that they carry the most oxygen-rich blood, because
they lead
to
the heart; just as the pulmonary artery carries the oxygen-poorest
blood away from the heart, to the lungs.) Like the right atrium, the
left atrium serves as a holding reservoir and, when full, releases its
contents into the left ventricle. A valve between left atrium and
ventricle closes, and the heart pumps.

To the Body

Blood surges through an opening valve of the left ventricle into the
aorta, the major artery that marks the beginning of blood's
circulation throughout the body. The left ventricle, because it has the
job of pumping blood to the entire body rather than just to the lungs,
is slightly larger and more muscular than the right ventricle. It is for
this reason, incidentally, that the heart is commonly considered to be
on our left. The organ as a whole, as noted earlier, is located at the
center of the chest.

The Heart's Own Circulatory System

Heart tissue, like that of every other organ in the body, must be
continually supplied with fresh, oxygen-rich blood, and used blood must
be returned to the lungs for reoxygenation. The blood inside the heart
cannot serve these needs. Thus the heart has its own circulation
network, called
coronary arteries
and
veins
, to nourish its muscular tissues. There are two major arteries on the
surface of the heart, branching and rebranching eventually into
capillaries. Coronary veins then take blood back to the right atrium.

Structure of the Heart

The musculature of the heart is called cardiac muscle because it is
different in appearance from the two other major types of muscle. The
heart muscle is sometimes considered as one anatomical unit, called the
myocardium
. A tough outer layer of membranous tissue, called the
pericardium
, surrounds the myocardium. Lining the internal chambers and valves of
the heart, on the walls of the atria and ventricles, is a tissue called
the
endocardium
.

These tissues, like any others, are subject to infections and other
disorders. An infection of the endocardium by bacteria is called
bacterial endocarditis
. (Disease of the valves is also called
endocarditis
, although it is a misnomer.) An interruption of the blood supply to the
heart muscle is called a
myocardial infarction
, which results in the weakening or death of the portion of the
myocardium whose blood supply is blocked. Fortunately, in many cases,
other blood vessels may eventually take over the job of supplying the
blood-starved area of heart muscle.

Heartbeat

The rate at which the heart beats is controlled by both the autonomic
nervous system and by hormones of the endocrine system. The precise
means by which the chambers and valves of the heart are made to work in
perfect coordination are not fully understood. It is known, however,
that the heart has one or more natural cardiac pacemakers that send
electrical waves through the heart, causing the opening and closing of
valves and muscular contraction, or pumping, of the ventricles near the
normal adult rate of about 72 times per minute.

One particular electrical impulse (there may be others) originates in a
small area in the upper part of the right atrium called the
sinus node
. Because it is definitely known that the contraction of the heart is
electrically activated, tiny battery-powered devices called
artificial pacemakers
have been developed that can take the place of a natural pacemaker
whose function has been impaired by heart injury or disease. Through
electrodes implanted in heart tissue, such devices supply the correct
beat for a defective heart. The bulk of the device is usually worn
outside the body or is implanted just under the skin.

The fact that both ventricles give their push at the same time is very
significant. It allows the entire heart muscle to rest between
contractions—a rest period that adds up to a little more than
half of a person's lifetime. Without this rest period, it is more
than likely that our hearts would wear out considerably sooner than they
do.

Blood Pressure

A physician's taking of blood pressure is based upon the
difference between the heart's action at its period of momentary
rest and at the moment of maximum work (the contraction or push). The
split-second of maximum work, at the peak of the ventricles’
contraction, is called the
systole
. The split-second of peak relaxation, when blood from the atria is
draining into and filling up the ventricles, is called the
diastole
.

Blood pressure measures the force with which blood is passing through a
major artery, such as one in the arm, and this pressure varies between a
higher
systolic
pressure, corresponding to the heart's systole, and a lower
diastolic
pressure, reflecting the heart's diastole, or resting phase. The
device with which a physician takes your blood pressure, called a
sphygmomanometer
, registers these higher and lower figures in numbers equivalent to the
number of millimeters the force of your arterial blood would raise a
column of mercury. The higher systolic force (pressure) is given first,
then the diastolic figure. For example, 125/80 is within the normal
range of blood pressure. Readings that are above the normal
range—and stay elevated over a period of time—indicate a
person has high blood pressure, or hypertension.

Hypertension has no direct connection with nervous tension, although the
two may be associated in the same person. What it does indicate is that
a heart is working harder than the average heart to push blood through
the system. In turn, this may indicate the presence of a circulatory
problem that might eventually endanger health. See also Ch. 9,
Diseases of the Circulatory System
and Ch. 10,
Heart Disease
.

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